From: Ian Wright (iwright@GMAIL.COM)
Date: Wed Sep 22 2004 - 21:03:24 EDT
Hi Paul
Thanks for the answer, which I think I understand. If correlation
assumes a special kind of vector space then I'd expect that to be
discussed in theoretical statistics. It should be a known issue. The
argument revolves around the following inference: "... this space is
not a vector space and it is questionable whether measures of
similarity based on vector space metrics are appropriate for it". If
it is questionable, then I'd guess some mathematicians will have made
it precise. It would seem important to get to the root of this one.
There's some confusion of terminology regarding vector spaces. Vector
spaces are not defined by their metric. A vector space with a metric
is a normed vector space. Euclidean space is one kind of normed vector
space. Manhattan distance will also yield a vector space. So I am
guessing that your references to "vector space" need to be replaced
with "Euclidean space" in order to convey your intention more
precisely. This confused me to begin with.
You label the metric actually observed in the space of bundles of
commodities d_{b} and go on to state: "Because of its metric, this
space is not a vector space ..." I agree that this space is not
Euclidean, but it may yet still be a vector space. First, we need to
check whether the observed metric d_{b}(p,0) satisfies the three
conditions for a norm. It is zero when p is zero. If we multiply p by
a scalar and get ap then the norm of ap is equal to a times the norm
of p. Finally, the metric must satisfy the triangle inequality, which
it does by equality. Therefore, I think it safe to conclude that the
observed metric is consistent with a vector space, contrary to what is
stated. But yes it is not Euclidean, so if the similarity measures are
based on an underlying Euclidean space then your argument proceeds
unaltered.
The observed metric is unusual because it has weights.
What is the ontological status of commodity amplitude space? Given
that the observed metric is consistent with a vector space then maybe
this suggests that the commodity amplitude space is a convenient
mapping to a Euclidean vector space, i.e. a useful re-representation
of the problem, rather than pointing to some hitherto unnoticed
ontology. (You state that commodity amplitude space is a "true vector
space", so I guess the norm is Euclidean, although I don't think you
state it). The analogy with quantum mechanics suggested to me that
maybe this is pointing to some hidden thing, but now I think maybe
not. The important thing is still the holdings, and the commodity
amplitudes are a convenient transformation of the situation.
I need to look it up, but my memory has been jogged that in some
geometries straight lines in fact represent circles, which is
suggestive, but this may be red-herring.
I think all this is interesting and stimulating. I always return to
the "Value's Law, Value's Metric" paper, basically because it
intrigues me and because I honestly am not sure what to think of it. I
feel there is something important there, but don't know what it is.
-Ian.
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